Texture mapping landscape
You can use the 3d View combined with the already unwrapped UV to paint the texture. The relevant part is:


  Paint the mesh in the 3D View, and let Blender use the currently selected UV map to update the UV Texture ("Projection Painting")


To do that, you first need to unwrap your object which you have done. Nice!

Second, you need to switch into texture paint mode


You will see on the right panel a message that says "Missing Data" You don't have anything to paint on yet! Select "Add Paint Slot" and choose Diffuse Color


From here you will be prompted to choose a size of the canvas, and to name it. I suggest you start with 2048 as it is easier to scale down than scale up. If you need to get up close and personal with your road, use a higher resolution.



We're almost at the fun part!

if you go to your properties window, you can see that a new material has been added to your object. Great! But, its all black. Not so great. To fix that we need to navigate one more tab to the right and see the Textures Tab.


Cool! I have renamed this base texture (the Canvas) as DiffuseRoad because we are building a road. However, we need another texture to paint ON to our canvas. To do this, select the texture box below and choose "New Texture". Open your road texture image in this texture and name it something useful: In my case, I only have assets of wood so I chose my wood texture and named it "PaintTexture" because thats what we will be doing with it!




From here, return to your side bar and select the "texture" red/white checker icon. Choose your texture, in my case "PaintTexture"


Now paint directly onto your 3d model! You should probably adjust the strength to 1 instead of .5, unless you do plan on mixing with a few textures. If you do need more textures, just go back to your Textures tab and add some more the same way as previously and select them.

Now select the DiffuseRoad texture that was created for us and choose it to be our UV Background, that way as you paint your model you can see in real time how your UV will look.




Once you are happy with how you have painted your model, be sure to save the UV as a separate image so you don`t lose it!